Generally, a piezoelectric material is a material capable of converting a mechanical energy to an electrical energy and vice versa. A typical example of a piezoelectric material is lead zirconate titanate having a perovskite crystalline structure (Pb(Zr,Ti)O3) (hereinafter referred to as “PZT”). In PZT, the greatest piezoelectric displacement is obtained in the <001> direction (the c axis direction) in the case of a tetragonal system, and in the <111> direction in the case of a rhombohedral system. However, many of the piezoelectric materials are polycrystals made up of a collection of crystal grains, and the crystallographic axes of the crystal grains are oriented randomly. Therefore, the spontaneous polarizations Ps are also arranged randomly.
Along with the recent downsizing of electronic appliances, there is a strong demand for reducing the size of piezoelectric elements using a piezoelectric material. In order to meet the demand, more piezoelectric elements are already used in the form of thin films whose volumes can be significantly reduced from those of sinters, which have conventionally been used in various applications, and active researches and developments have been made for reducing the thickness of thin-film piezoelectric elements. For example, in the case of tetragonal PZT, the spontaneous polarization Ps is oriented in the c axis direction. Therefore, in order to realize superior piezoelectric characteristics even with a reduced thickness, the c axes of crystal grains forming a PZT thin film need to be aligned vertical to the substrate plane. In order to realize such an alignment, a sputtering method has been used in the prior art. Specifically, on a single crystal substrate made of magnesium oxide (MgO) having an NaCl-type crystalline structure, which has been cut out so that the surface thereof is along the crystal orientation of the (100) plane, a (100)-oriented Pt electrode thin film is formed as a lower electrode on the substrate, and a PZT thin film whose c axis is oriented vertical to the surface of the Pt electrode is formed on the Pt electrode at a temperature of 600 to 700° C. (see, for example, Journal of Applied Physics vol. 65 No. 4 (published on 15 Feb. 1989 from the American Physical Society) pp. 1666-1670, and Japanese Unexamined Patent Publication No. 10-209517). In such a case, if a piezoelectric element layer having a thickness of 0.1 μm and made of PbTiO3 or (Pb,La)TiO3, free of Zr, is formed as a base layer for a PZT thin film on the (100)-oriented Pt electrode before the formation of a PZT thin film, and then a PZT thin film having a thickness of 2.5 μm is formed on the piezoelectric element layer by a sputtering method, it is less likely that a layer of a low crystallinity made of a Zr oxide is formed early in the formation of the PZT thin film, thereby obtaining a PZT thin film having a higher crystallinity. Specifically, a PZT thin film whose degree of (001) orientation (“α(001)”) is about 100% is obtained.
Herein, α(001) is defined as follows:α(001)=I(001)/ΣI(hkl)
ΣI(hkl) is the sum of diffraction peak intensities from various crystal planes of PZT having a perovskite crystalline structure for a Cu—Kα 2θ range of 10° to 70° in an X-ray diffraction method. Note that the (002) plane and the (200) plane are not included in ΣI(hkl) as they are equivalent to the (001) plane and (100) plane.
However, this method uses an MgO single crystal substrate as a base substrate, thereby increasing the cost of a piezoelectric element, and thus the cost of an ink jet head using the piezoelectric element. Moreover, another drawback is that the variety of the substrate material is limited to the MgO single crystal.
In view of this, various methods have been developed for forming a (001)- or (100)-oriented film of a perovskite piezoelectric material such as PZT on an inexpensive substrate such as a silicon substrate. For example, Japanese Patent Publication No. 3021930 discloses that a PZT film that is preferentially oriented along the (100) plane can be produced by applying a precursor solution of PZT or lanthanum-containing PZT on a (111)-oriented Pt electrode, performing a thermal decomposition process at 450 to 550° C. before the precursor solution is crystallized and then heating and crystallizing the precursor solution at 550 to 800° C. (a sol-gel method).
Moreover, Japanese Unexamined Patent Publication No. 2001-88294 discloses that by forming a very thin titanium layer on an iridium lower electrode, it is possible to control the crystal orientation of a PZT film to be formed thereon. This manufacturing method includes: forming a base layer whose main component is zirconium oxide on a substrate made of silicon, or the like; forming a lower electrode containing iridium on the base layer; depositing a very thin titanium layer on the lower electrode; forming an amorphous piezoelectric precursor thin film containing metal element and oxygen element, which forms a ferroelectric having piezoelectric characteristics, on the titanium layer; and heating and crystallizing the amorphous thin film at a high temperature (a sol-gel method), thereby turning the amorphous thin film into a perovskite piezoelectric thin film. With this manufacturing method, the crystal orientation of the piezoelectric thin film such as PZT can be controlled by the thickness of the titanium layer, and a (100)-oriented film is obtained when the thickness of the titanium layer is set to be 2 to 10 nm.
Furthermore, Japanese Unexamined Patent Publication No. 11-191646 discloses that where a piezoelectric thin film is formed by using a sol-gel method, a (100)-oriented PZT film can be obtained by forming a titanium layer having a thickness of 4 to 6 nm on a (111)-oriented Pt electrode and using titanium oxide, which is formed through oxidization of titanium in the titanium layer, as a nucleus.
However, while the methods described above are desirable methods that do not use an expensive MgO single-crystal substrate, it is difficult to obtain a well-oriented film having a desirable crystallinity in the film formation process, as in the case of forming a piezoelectric thin film on an MgO single-crystal substrate, because the piezoelectric thin film is formed by a sol-gel method. In view of this, an amorphous piezoelectric thin film is first formed, and then the layered structure including the substrate and the piezoelectric thin film is subjected to a heat treatment, so that the crystallographic axes are preferentially oriented in a desirable direction.
Moreover, when piezoelectric elements are mass-produced with a sol-gel method, the amorphous piezoelectric precursor thin film is likely to be cracked due to changes in the volume during the degreasing step of removing organic substances. Furthermore, in the step of heating and crystallizing the amorphous piezoelectric precursor thin film at a high temperature, the film is likely to be cracked or peeled off from the lower electrode due to crystal changes.
As a solution to these problems with a sol-gel method, Japanese Unexamined Patent Publication Nos. 2000-252544 and 10-81016 disclose that it is effective to add titanium or titanium oxide in the lower electrode. Particularly, Japanese Unexamined Patent Publication No. 10-81016 shows that a (100)-oriented PZT film can be obtained even with a sputtering method. Note however that a perovskite PZT film is not obtained directly on the lower electrode. First, a PZT film having an amorphous or pyrochlore crystalline structure is formed at a low temperature of 200° C. or less, which is then crystallized through a heat treatment at a high temperature of 500 to 700° C. in an oxygen atmosphere. Therefore, as with a sol-gel method, the film is likely to be cracked or peeled off from the lower electrode due to crystal changes in the step of heating and crystallizing the film at a high temperature. Moreover, the degree of (001) orientation or the degree of (100) orientation of the PZT film formed by a sol-gel method or a sputtering method as described above is 85% or less with either method.
Furthermore, with a sol-gel method, the maximum thickness of the PZT film to be formed in a single iteration of the step (including the application of the precursor solution and the following heat treatment) is about 100 nm at maximum. Therefore, in order to obtain a thickness of 1 μm or more, which is required for a piezoelectric element, it is necessary to repeat this step ten times or more, whereby the production yield may be reduced.
On the other hand, according to Japanese Unexamined Patent Publication No. 2001-88294, supra, states that attempts were made to control the orientation of PZT on an Ir base electrode with a very thin titanium layer formed thereon by using a method other than a sol-gel method (including an MOD method) (in which an amorphous thin film is once formed and then the thin film is turned into a crystalline thin film through an aftertreatment such as a heat treatment), i.e., by using a method in which a crystalline thin film is directly formed without the crystallization step using a heat treatment, e.g., a sputtering method, a laser ablation method or a CVD method, and that a well-oriented film was not obtained with any method other than a sol-gel method. The reason is stated to be as follows. The crystallization of the PZT film proceeds gradually from the lower electrode side to the upper electrode side with a sol-gel method, whereas with a CVD method or a sputtering method, the crystallization of the PZT film proceeds randomly, resulting in irregular crystallization, and thus making the orientation control difficult.
Moreover, when a titanium oxide film whose thickness is 12 nm or less is formed on a (111)-oriented Pt electrode layer, and a lead titanate film or a PZT film having a perovskite crystalline structure is formed directly by a sputtering method, either film exhibits a (111) orientation property, and a (100)- or (001)-oriented film is not obtained (see Journal of Applied Physics vol. 83 No. 7 (published on 1 Apr. 1998 from the American Physical Society) pp. 3835-3841).
The present invention has been made in view of the above, and has an object to provide a reliable piezoelectric element with desirable piezoelectric characteristics at low cost.